Greasy pots

For archaeologists it is a good thing that ancient cooks sometimes forgot to wash up.

Fats and oils from cooking animals or dairy products are known to survive for thousands of years if they have been absorbed in to porous ceramic pots. The pot protects the molecules from water and microbes.

The molecules that survive for thousands of years are fatty acids. These are long chain organic molecules made from carbon, hydrogen and oxygen. They have long hydrocarbon chains. Fatty acids are named according to the number of carbon atoms they have. So C16 (n-hexadecanoic acid) has 16 carbon atoms and C18 (n-octadecanoic acid) has 18 carbon atoms.

BE CAREFUL. C16 and C18 look like isotopes of carbon. They are not. These are the numbers of carbon atoms in the fatty acid molecule. This can be confusing because we will be talking about analyzing the ratio of 13C and 12C in these carbon atoms. So we might say that we want to measure the amount of 13C in C16 molecules. That means how much of the carbon isotope 13C is in the fatty acid molecules with 16 carbon atoms each. This is precisely what we will be doing further down the page, so keep concentrating!

Fig. 7.4 3D models of some fatty acids

Getting the fatty acids out of pottery

Fig. 7.5 Gas chromatography separation of fatty acid molecules.

Fig. 7.5 shows how three different sized molecules are separated as they travel through the gas column. The molecules that stick least to the stationary phase get though first. It is a race to the far end of the tube. So the GC machine produces a graph showing the time each group of molecules came out.

For fatty acids we can get a graph like this.

Fig. 7.6 Gas chromatograph showing two main fatty acids, C16 (16 carbon atoms) and C18 (18 carbon atoms) and smaller peaks for C14, 15 and 17. The x-axis shows how long each fatty acid molecule took to get through the column.

And now the clever part!

The GC machine separates out the different fatty acid molecules. It can now automatically send them to a mass spectrometer or an isotope ratio mass spectrometer. In the isotope ratio mass spectrometer the samples are burnt in a micro furnace to make CO2. The CO2 molecules are ionised to give them a charge, q, and sent through the mass spectrometer. The difference in the mass, m, shows up when the ratio m/q is measured to an accuracy of better than 0.5 parts per million. The difference of one neutron can be resolved.

So we can get this type of data. Remember that the 13C is the carbon 13 isotope (the fishy isotope). 16 and 18 are the numbers of carbon atoms in the fatty acids.

Fig. 7.7 Scatter plot of carbon 13 for several animal fats. These are from present day animals. The isotope levels are from fatty acids 16 and 18 separated by gas chromatography as shown in Fig. 7.6

Fig. 7.7 shows that there is not much difference between the carbon 13 ratio between fatty acid 16 and fatty acid 18; after all they come from the same animal for each data point. On the scatter plot this makes a 45 degree line. However there is one result slightly off the 45 degree line. Ruminant adipose (fat from the meat of ruminant animals) and ruminant milk.
There is a complex biological reason for the milk fats having less carbon 13 in the fatty acid 18. The important thing about this discovery is that ancient pots can reveal what was cooked in them. Dairy fats from ruminant milk fall in a unique place on the chart.

With these techniques archaeologists can identify where and when dairying was happening and where dairy products were cooked.

Fig. 7.8 Analysis of C16 and C18 lipids enables archaeologists to pinpoint where dairy products and meat products were cooked.

Fig. 7.8 shows a map of Durrington Walls, a Neolithic village discovered near Stonehenge. It appears that dairy products were cooked in a communal area, while meat products were cooked in an area where houses were built.

Fig.7.2 Paper chromatography separates molecules from a mixture. You may have done this with ink at school using filter paper.
The paper is called the "stationary phase". The water carrying the molecules is the "mobile phase".

Fig.7.3 In gas chromatography (GC) the stationary phase is a long fine silica tube with a special coating inside. This one is 60 metres long. The coating is the stationary phase and gas is the mobile phase.